The degree of acidity or alkalinity of the water is important. A high pH over 7.8, causes scaling and decreases the efficiency of chlorine. A low pH under 7.2, causes corrosion of the metal content in pool water and of metal equipment in contact with  pool water (in particular where more than one metal is present) and increases the possibility of irritation to the nose, throat and eyes of bathers. The pH is especially important when pool water is recirculated through a heater. The ideal pH is 7.2 to 7.8. The pH level should always be adjusted before adding chlorine to the pool water.